Related Work.
FL is a fast evolving topic, and we only describe closely related approaches here. Comprehensive field studies have appeared in [1]}, [2]}. The general FL setup involves two types of updates, the server and device, and each of these updates are associated with minimizing some local loss function, which by itself could be updated dynamically over different rounds. At any round, there are methods that attempt to fully optimize or others that propose inexact optimization. We specifically focus on relevant works that address the four FL scenarios (massive distribution, heterogeneity, unreliable links, and unbalanced data) here.
